A series of experiments are proposed to test and validate the idea, that mouse mammary fibroblasts (mMgF) will provide an equivalent microenvironment for proper growth and differentiation of human breast epithelium (huBrE). Definitive proof of this idea raises the possibility of using mutant mouse mammary gland stroma to investigate paracrine pathways affecting huBrE. In addition, we will assess the effects of carcinoma-associated mammary fibroblasts (CAF) on the growth, differentiation, and ductal morphogenesis of huBrE. It is anticipated that this research program will lead to a mechanistic understanding at the cellular and molecular levels of stromal-epithelial interactions in growth, differentiation, and ductal morphogenesis of huBrE and how changes in the stroma lead to the initial stages predisposing to breast cancer. Such research may lead to the discovery of new markers for stratification of breast cancer patients and possibly new therapies. To accomplish these goals, 5 specific aims are proposed. Specific Aim #1 is an analysis of differentiation and growth of normal human breast epithelium grown in vivo in combination with fibroblasts derived from normal mouse and human mammary gland. Specific Aim #2 uses tissue recombinants composed of mutant mouse mammary fibroblasts plus human breast epithelial cells to assess the role of paracrine pathways regulating growth and differentiation of human breast epithelium. In this specific aim we will assess the role of stromal estrogen receptors and "EGF-signaling" in growth, differentiation, and ductal morphogenesis of human breast epithelium. Specific Aim #3 is a characterization of differentiation and growth response of human breast epithelium grown in association with carcinoma-associated mammary fibroblasts (CAF) of mouse and human origin focusing specifically on CAF effects on (a) epithelial differentiation and (b) growth response. Specific Aim #4 focuses on the effects of CAF on E-cadherin (E-cad) and integrin expression in human breast epithelium in vivo and vitro. We will specifically determine whether perturbation of epithelial differentiation by CAF is due to perturbation of adhesion systems such as E- and P-cadherin, intergrins or the basement membrane. Specific Aim #5 deals with identification of molecular markers unique to CAF and an investigation of the time course of change in epithelial gene expression elicited by CAF in target epithelium. To accomplish these goals we will use tissue recombinant technology, cell and organ culture, immunocytochemistry, and various molecular methodologies.